title: Naringin directly activates inwardly rectifying potassium channels at an overlapping binding site to tertiapin-Q.
abstract: BACKGROUND
  G protein-coupled inwardly rectifying potassium (K(IR) 3) channels are important proteins that regulate numerous physiological processes including excitatory responses in the CNS and the control of heart rate. Flavonoids have been shown to have significant health benefits and are a diverse source of compounds for identifying agents with novel mechanisms of action.
  
  
  METHODS
  The flavonoid glycoside, naringin, was evaluated on recombinant human K(IR) 3.1-3.4 and K(IR) 3.1-3.2 expressed in Xenopus oocytes using two-electrode voltage clamp methods. In addition, we evaluated the activity of naringin alone and in the presence of the K(IR) 3 channel blocker tertiapin-Q (0.5 nM, 1 nM and 3 nM) at recombinant K(IR) 3.1-3.4 channels. Site-directed mutagenesis was used to identify amino acids within the M1-M2 loop of the K(IR) 3.1(F137S) mutant channel important for naringin's activity.
  
  
  RESULTS
  Naringin (100 µM) had minimal effect on uninjected oocytes but activated K(IR) 3.1-3.4 and K(IR) 3.1-3.2 channels. The activation by naringin of K(IR) 3.1-3.4 channels was inhibited by tertiapin-Q in a competitive manner. An alanine-scan performed on the K(IR) 3.1(F137S) mutant channel, replacing one by one aromatic amino acids within the M1-M2 loop, identified tyrosines 148 and 150 to be significantly contributing to the affinity of naringin as these mutations reduced the activity of naringin by 20- and 40-fold respectively.
  
  
  CONCLUSIONS
  These results show that naringin is a direct activator of K(IR) 3 channels and that tertiapin-Q shares an overlapping binding site on the K(IR) 3.1-3.4. This is the first example of a ligand that activates K(IR) 3 channels by binding to the extracellular M1-M2 linker of the channel.
authors.affiliation: Faculty of Pharmacy, The University of Sydney, Sydney, NSW, Australia.
authors.forename: Tin T
authors.initials: TT
authors.lastname: Yow
authors.forename: Elena
authors.initials: E
authors.lastname: Pera
authors.forename: Nathan
authors.initials: N
authors.lastname: Absalom
authors.forename: Marika
authors.initials: M
authors.lastname: Heblinski
authors.forename: Graham A R
authors.initials: GA
authors.lastname: Johnston
authors.forename: Jane R
authors.initials: JR
authors.lastname: Hanrahan
authors.forename: Mary
authors.initials: M
authors.lastname: Chebib
mesh_terms: D000818:Animals; D001514:Bee Venoms / Q000494:pharmacology*; D001665:Binding Sites; D001667:Binding, Competitive; D002478:Cells, Cultured; D044950:Flavanones / Q000737:chemistry / Q000494:pharmacology*; D006801:Humans; D015394:Molecular Structure; D016297:Mutagenesis, Site-Directed; D009865:Oocytes; D024661:Potassium Channels, Inwardly Rectifying / Q000819:agonists* / Q000037:antagonists & inhibitors / Q000235:genetics; D011485:Protein Binding; D011994:Recombinant Proteins / Q000819:agonists / Q000037:antagonists & inhibitors / Q000235:genetics; D014162:Transfection; D014982:Xenopus laevis
chemical_list: D001514:Bee Venoms; D044950:Flavanones; D024661:Potassium Channels, Inwardly Rectifying; D011994:Recombinant Proteins; C029738:tertiapin; C005274:naringin